The neural regulation of mastication is poorly understood. A central pattern generator (CPG) in the brainstem controls the basic features of the automatic chewing pattern while sensory feedback modulates this rhythm. We hypothesize that commands controlling voluntary jaw movements by-pass the CPG. Four Protocols (PI-PIV) are proposed to test this hypothesis. Adults with no discernible oro-facial dysfunctions and patients with evidence of TMJ problems will be asked to chew standard size pieces of flavorless gum of five chewing toughnesses. In PI subject will chew each hardness automatically while his attention is distracted. A Kinesiograph will record mandibular movements; surface electrodes will detect gross masseter activity; fine wires implanted in the masseter will detect single motor unit (SMU) potentials. Multiple variables will be measured from each record on a cycle-by-cycle basis to assess quantitatively masticatory output. Cycle duration, opening and closing velocities and the duration of centric occlusion will be derived from the Kinesiograph. The surface electromyogram will be integrated and each cyclic burst of masseter activity will be analyzed. From spike-by-spike analysis of SMU recordings the behavior of masseteric motoneurons will be deduced. In PII subjects will chew voluntarily in time with a metronome set at a frequency approximating the subject's mean automatic chewing frequency. Differences between corresponding variables obtained during PI and PII will reveal differences between automatic and voluntary control. In PIII, the voluntary chewing pattern will be studied at different frequencies to determine which variables are modulated by CNS control. PIV is similar to PI except local anesthetic will be injected around two upper and two lower molars to eliminate sensory feedback. Differences between PI and PIV variables will reveal the modulatory action of periodontal feedback. In the third year 30 patients with objective evidence of TMJ dysfunction will be similarly studied. This project promises to provide missing information about the functional organization of the motor system regulating jaw movements and masseteric motoneurons during automatic and voluntary chewing. It should also reveal how TMJ dysfunction affects the system's motor output.